The invention relates to a platform structure intended to rest on the bottom of the sea or other body of water and to support installations generally assembled on a platform as high as possible above water level from some industrial or scientific organization, such as for exploration, oil production, or an electrical generating station. The handling operations are carried out with the aid of a crane.
Such a platform structure generally consists of a base resting on the bottom of the sea to support a platform comprising one or a multiplicity of bridges, intended for the installations, at a considerable height above the water, for example, 40 meters or more. An essential elements of this installation is the crane which can be used in the construction of the platform structure. Thus, it is one of the first items installed on the structure resting on the underwater bed. Preferably, the crane is generally located at a point which is offset with respect to the central shaft intended to support the platform, and in such a manner that its boom can extend over all parts of the platform and raise installations brought by boats which do not have to come too close to the platform.
Thus, this invention is restricted to cranes installed on the finished platform before the platform is placed on the base. However, for reasons of stability and because of the dynamic forces created during transport, the crane cannot be placed at its operational height which is very much above the level of the platform.
Thus, the crane is positively fixed on its support which, in its turn, is fixed to the platform in a temporary manner. This temporary relationship is not disturbed until the platform and the base have been brought together.
The installation of a self-raising platform with its vertical support- and guide-rails on the base of a structure has already been envisaged for the construction of works at sea which have to feature a crane mounted on a column which dominates the superstructure. The head of the crane support column is constructed subsequently through a hole in this platform and the crane is mounted on it. Afterward, the platform and the crane are raised in keeping with the construction to the height of the works and the column.